1. Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to an improved front assembly for color tubes having shadow masks of the tension foil type in association with a substantially flat faceplate. The invention is useful in color tubes of various types, including those used in home entertainment television receivers, and in medium-resolution and high-resolution tubes intended for color monitors.
The use of the foil-type flat tension mask and flat faceplate provides many benefits in comparison to the conventional domed shadow mask and correlatively curved faceplate. Chief among these is a greater power-handling capability which makes possible as much as a three-fold increase in brightness. The conventional curved shadow mask, which is not under tension, tends to "dome" in picture areas of high brightness where the intensity of the electron beam bombardment is greatest. Color impurities result as the mask moves closer to the faceplate and as the beam-passing apertures move out of registration with their associated phosphor elements on the faceplate. The tension mask when heated distorts in a manner quite different from the conventional mask. If the entire mask is heated uniformly, there is no doming and no distortion until tension is completely lost; just before that point, wrinkling may occur in the corners. If only portions of the mask are heated, those portions expand, and the unheated portions contract, resulting in displacements within the plane of the mask; i.e., the mask remains flat.
The tension foil shadow mask is a part of the cathod ray tube front assembly, and is located in close adjacency to the faceplate. The front assembly comprises the faceplate with its screen consisting of deposits of light-emitting phosphors, a shadow mask, and support means for the mask. As used herein, the term "shadow mask" means an apertured metallic foil which may, by way of example, be about 0.001 inch thick, or less. The mask must be supported in high tension a predetermined distance from the inner surface of the cathode ray tube faceplate; this distance is known as the "Q-distance." As is well known in the art, the shadow mask acts as a color-selection electrode, or parallax barrier, which ensures that each of the three beams lands only on its assigned phosphor deposits.
The requirements for a support means for a foil shadow masks mask are stringent. As has been noted, the foil shadow mask is normally mounted under high tension. The support means must be of high strength so the mask is held immovable; an inward movement of the mask of as little as 0.0002 inch can cause the loss of guard band. Also, it is desirable that the shadow mask support means be of such configuration and material composition as to be compatible with the means to which it is attached. As an example, if the support means is attached to glass, such as the glass of the inner surface of the faceplate, the support means must have a coefficient of thermal expansion compatible with the glass, and by its composition, be bondable to glass. Also, the support means should be of such composition and structure that the mask can be secured to it by production-worthy techniques such as electrical resistance welding or laser welding. Further, it is essential that the support means provide a suitable surface for mounting and securing the mask. The material of which the surface is composed should be adaptable to machining or other forms of shaping so that it can be contoured into near-perfect flatness so that no voids between the metal of the mask and the support structure can exist to prevent the positive, all-over contact required for proper mask securement.
2. Prior Art
An avionics color cathode ray tube having ceramic components is described in a journal article by Robinder et al of Tektronix, Inc. A shadow mask is mounted in a ceramic ring/faceplate assembly, with the mask suspended by four springs oriented in the z-axis. Ceramic is also used to form a two-piece, x-ray-attenuating body. A flat faceplate is utilized, together with a glass neck flare. (From "A High-Brightness Shadow-Mask Color CRT for Cockpit Displays," Robinder et al. Digest of a paper presented at the 1983 symposium, Society for Information Display.)
A color picture tube having a conventional curved faceplate and correlatively curved, untensed shadow mask is disclosed in Japanese Patent No. 56-141148 to Mitsuru Matshusita. The purpose according to a quotation from the abstract is ". . . To rationalize construction and assembly of a tube, by both constituting its envelope from a panel, ceramic shadow mask mounting frame and funnel and integrally forming a surplus electron beam shielding plate to the shadow mask mounting frame."
3. Other Prior Art
A journal article: "The CBS Colortron: A Color Picture Tube of Advanced Design." Fyler et al. Proceedings of the Institute of Radio Engineers (IRE), Jan. 1954.